1. Field of the Invention
This invention relates to a method for monitoring and controlling the burner operating air equivalence ratio in an industrial combustion apparatus and to a monitoring and control apparatus for implementing this method.
2. Prior Art Statement
In an industrial combustion apparatus, fuel and air are supplied to and burned in one or more burners so as to utilize the resulting thermal energy. When the ratio of the amount of supplied air to the amount of air theoretically required for complete combustion (this ratio being called the "burner operating air equivalence ratio" hereinafter) is less than unity, fuel fails to burn completely and unburnt fuel remains. As it is therefore impossible to realize complete conversion of the fuel's chemical energy into thermal energy, energy loss results. On the other hand, when the burner operating air equivalence ratio is greater than unity, the amount of combustion gas increases in proportion to the air equivalence ratio, which results in an increase in heat lost by being carried away by the flue gas and a corresponding decline in thermal efficiency. Thus for achieving maximum thermal efficiency, it is necessary to burn fuel under the smallest air equivalence ratio possible within the range which does not lead to incomplete combustion. This optimum air equivalence ratio is determined by fuel-air mixing characteristics in the burner flame and thus depends on the burner structure, the kind of fuel, heat release rate and the like. Therefore, the optimum air equivalence ratio is not a universal value among a wide variety of combustion apparatus. This makes it necessary to monitor the air equivalence ratio at each of the burners with high accuracy so that the burner can be controlled and maintained at the desired air equivalence ratio.
The method for monitoring and controlling air equivalence ratio which has been generally used is as follows. The overall air equivalence ratio in the combustion apparatus is computed from the combustion gas composition which is analyzed through flue gas sampling, and the computed air equivalence ratio is fed back to the operation for controlling the air equivalence ratio (the air flow rate adjustment operation).
However, this conventional monitoring and control method is disadvantageous in that, for example, (1) the burner operating air equivalence ratio cannot be accurately detected because the flue gas composition does not represent the actual burner operating air equivalence ratio when atmospheric air leaks into the apparatus, and (2) most industrial combustion apparatus are equipped with more than one burner so that even though the overall air equivalence ratio may be appropriate for the apparatus as a whole, this does not necessarily mean that each individual the burner is operated under optimum air equivalence ratio.
For overcoming these problems, there have been proposed two kinds of methods for monitoring and controlling the operating air equivalence ratio of the individual burners based on detecting the light emission from the flame. These are summarized here.
(1) The burner operating air equivalence ratio is controlled so as to maintain the intensity of some specific emission spectrum from the flame at its maximum value. This method is based on the general combustion characteristics that when the air equivalence ratio is in the vicinity of unity, fuel and air react most vigorously, causing the maximum intensity of the flame emission spectra.
(2) The ratio between the intensities of two specific emission spectra with different wavelength from the flame varies in a specific manner with change in the air equivalence ratio. Based on this characteristic, control is conducted by maintaining the ratio between two specific emission spectra at that corresponding to the target air equivalence ratio.
The first of these methods has the defect that the air equivalence ratio can be controlled only to a fixed value in the vicinity of unity and thus cannot be arbitrarily adjusted to the optimum value particular to the combustion apparatus in use. The second method has the disadvantage that the flame emission spectra in the visible and/or infrared regions employed as indices can be affected by the radiation from hot portions in the apparatus such as the burner tile and the furnace wall. This gives rise to problems regarding accuracy and stability. Moreover, since the variation of the spectral intensity ratio with air equivalence ratio is not an algebraic function, it is necessary to use a complex process for setting the target air equivalence ratio as well as a complex system for control.
Because of these defects in the earlier proposed methods and apparatuses, there has not yet been established technology enabling the light emission from the burner flame to be used for monitoring and controlling the burner operating air equivalence ratio.